Chromatic Dispersion Compensation by Signal Predistortion: Linear and Nonlinear Filtering

Chromatic dispersion is one of the most significant impairments in long-distance fiber-optical communications. Electrical domain compensation of optical chromatic dispersion by signal predistortion using digital processing is investigated with two different approaches: using a linear digital FIR compensating filter and using a nonlinear filter based on a look-up table (LUT). The linear compensating system is simulated with quadrature phase shift keying (QPSK) and 16-quadrature amplitude modulation (16-QAM) schemes. In the 16-QAM system the maximum amplitude of the signal generated after the compensating filter can be large, so the maximum amplitude is calculated and is employed in compensating the cosine function of the Cartesian Mach-Zehnder modulator. The time and frequency domain implementation of the linear compensating filter are considered more carefully and the number of adjacent symbols which contribute to the intersymbol interference is calculated. In [1] it has been shown that the LUT generated for compensating chromatic dispersion can be significantly compressed with the Hadamard transform. Nevertheless there are still some constraints in implementing such a system. These constraints, which are mainly memory constraints and processing time constraints, are introduced and some implementation details are discussed to avoid the memory constraints, e.g., the need for generating the large-size LUT before generating the compressed LUT. Removing this constraint allows us to go for longer input symbol sequences to the LUT. For example at the distance of 500 km in a 10 Gbaud/sec QPSK system, the chromatic dispersion can be completely compensated by taking 11 symbols as the input to the LUT. It is also shown that using at least a 4-bit digital-to-analog converter (DAC) in a QPSK system and a 5-bit DAC in a 16-QAM system does not lead to degradation of their performances. The linear compensating filter can compensate any amount of dispersion in both modulation schemes with a reasonable number of filter taps. The LUT-based compensating system, with or without the Hadamard transform, has the same performance as the linear system, i.e., it can compensate CD completely, if the length of the input symbol stream to the LUT is chosen at least equal to the number of adjacent symbols which contribute to the intersymbol interference. For shorter input sequences to the LUT, we see degradation in the performance of the system, which can now be improved by the Hadamard transform, as previously shown in [1].